The inclusion of thermal storage into a residential heat pump system provides an opportunity for electrical load shifting in order to reduce peak electricity demand. To assess this potential, a detailed numerical system model for a ground-source heat pump coupled with thermal storage was developed for residential heating applications. Water-based thermal storage and hybrid storage containing water and phase change materials (PCMs) were considered. The amount of thermal buffering needed to shift the heat pump operation to off peak electricity periods was numerically assessed for a house of 180 m(2) floor area in Toronto, Ontario, Canada. A very cold day ambient profile was considered as the extreme case to reach the study conclusions. Results indicated that total electrical load shift to off-peak hours was achieved by a 2.5 m(3) water tank or a 1 m(3) hybrid tank containing 50% PCM by volume. This implies around 65% reduction in storage volume without compromising the space heating capability. This is attributed to the higher storage capacity offered by the hybrid system when the temperature operating range is limited by heat pump operation. The influence of operating temperature ranges and packing ratio was presented. Lower operating ranges and higher packing ratios lead to better thermal buffering of the hybrid system at smaller storage volumes. However, careful choice of the PCM encapsulation geometry is needed to guarantee complete melting/solidification during the charging/discharging period. (C) 2019 Elsevier B.V. All rights reserved.